Expansion reservoir for a coolant circuit, having at least one inlet port and an outlet port, and a fill opening that can be closed by a lid, wherein a differential-pressure-controlled valve with at least two switching positions is integrated in the lid.
Such expansion reservoirs are employed in coolant circuits of internal combustion engines, in particular for motor vehicles, for separating air from the coolant circuit, for compensating for the increase in volume of the cooling medium being heated, for filling the coolant circuit and for building up a positive pressure to prevent the coolant from boiling.
DE 40 39 993 A1 discloses a vent line in the cooling circuit of an internal combustion engine, wherein the vent line connects a radiator located in an upper portion of the expansion reservoir and having a ceiling higher than the expansion reservoir. The expansion reservoir has in the ceiling a filler neck sealable by a lid, wherein the lid includes a pressure relief valve. A tubular element of the vent line dips into the expansion reservoir at the upper portion and has an opening in the vicinity of the reservoir bottom. The tubular element has in the region of its highest point in the expansion reservoir a port that is open to the atmosphere, wherein the port is formed by closed space in the filler neck that is closed towards the interior space of the expansion reservoir, which is closed off to the atmosphere when the filler neck is closed. The air that is displaced when the expansion reservoir and the radiator are filled can then escape via the open space through the filler neck. During the driving operation, the air is forced via the space to the lid when the filler neck is closed, wherein the lid is raised when a pressure that depends on the type of the lid is exceeded, thereby allowing the expansion reservoir to be vented to the atmosphere.
Disadvantageously, the that the vent line of such an expansion reservoir cannot be reliably closed during warm-up of the cooling circuit, so that unwanted heat may be introduced from the cooling circuit into the expansion reservoir. Furthermore, the expansion reservoir is unsuitable for cooling circuits that are continuously operating under overpressure, because venting to the atmosphere occurs from a certain pressure threshold on, causing the overpressure to decrease.